The invention herein disclosed pertains to a method and apparatus for testing the productivity of formations which contain petroleum products. The invention is particularly useful in the testing of offshore wells where it is desirable to conduct a testing program with a minimum of testing string manipulation, and preferably with the blowout preventers closed during a major portion of the program.
It is known in the art that sampler valves and tester valves for testing the productivity of oil wells may be operated by applying pressure increases to the fluid in the annulus of the well. For instance, U.S. Pat. No. 3,664,415 to Wray et al. discloses a sampler valve which is operated by applying annulus pressure increases against a piston in opposition to a predetermined charge of inert gas. When the annulus pressure overcomes the gas pressure, the piston moves to open a sampler valve thereby allowing formation fluid to flow into a sample chamber contained within the tool, and into the testing string facilitating production measurements and testing.
U.S. Pat. No. 3,858,649 to Holden et al. also discloses a sampler apparatus which is opened and closed by applying pressure changes to the fluid in the well annulus. This apparatus contains a supplementing means wherein the inert gas pressure is supplemented by the hydrostatic pressure of the fluid in the well annulus as the testing string is lowered into the borehole. This feature allows the use of lower inert gas pressure at the surface and provides that the gas pressure will automatically be adjusted in accordance with the hydrostatic pressure and environment at the testing depth, thereby avoiding complicated gas pressure calculations required by the earlier devices for proper operation. U.S. Pat. No. 3,856,085 to Holden et al. likewise provides a supplementing means for the inert gas pressure in a full opening testing apparatus.
The above mentioned supplementing means includes a floating piston exposed on one side to the inert gas pressure and on the second side to the annulus pressure in order that fluid pressure in the annulus can act on the gas pressure. The system is balanced to hold the valve in its normal position until the testing depth is reached. Upon reaching the testing depth, the floating piston is isolated from the annulus pressure so that subsequent changes in the annulus pressure will operate the particular valve concerned.
The prior method of isolating the floating piston has been to close the flow channel from the annulus to the floating piston with a valve which closes upon the addition of weight to the string. This is done by setting the string down on a packer which supports the string and isolates the formation during the test. The prior apparatus is designed to prevent the isolation valve from closing prematurely due to increasingly higher pressures as the test string is lowered into the well, contains means to transmit the motion necessary to actuate the packer mentioned above, and is designed to remain open until sufficient weight is set down on the packer to prevent premature isolation of the gas pressure and thus premature operation of the tester valve being used.
The present invention comprises a method for isolating the gas pressure from the fluid pressure in the annulus responsive to an increase in the annulus pressure by a predetermined amount above a reference pressure for use in an annulus pressure operated tool, wherein the operating force of the tool is supplied by the pressure of a gas in an inert gas chamber in the tool. The reference pressure used is the pressure which is present in the annulus at the time a well bore sealing packer is set.
The annulus pressure is allowed to communicate with an interior bore of the disclosed apparatus as the testing string is lowered in the well bore. This pressure is trapped as the above mentioned reference pressure when the packer seals off the well bore and isolates the formation to be tested. Subsequent increases in the well annulus pressure above the reference pressure activates a pressure responsive valve to isolate the inert gas pressure from the well annulus pressure. Additional pressure increases in the well annulus causes the well testing apparatus to operate in the conventional manner.
The invention disclosed simplifies the design and construction of the well testing apparatus. The resulting isolation valve is simple and has a minimum number of parts. The testing apparatus utilizing the invention of this disclosure will not have a discontinuity in its housing such as a collapsing section used to close the previously known isolating valves. A simplified isolating valve thus results which does not require special provision to transmit the movement necessary to set the packer, nor to support the forces of the drill string during the lowering or withdrawal of the test string in the borehole.